1. Field of the Invention
The present invention relates to a semiconductor device having capacitor elements, as well as to a process for producing the device. More particularly, the present invention relates to the structure of a high-dielectric-constant film or a ferroelectric film, both usable in capacitors of semiconductor integrated circuit and obtained by using organometal gases as the raw material, as well as to a method for forming such a film.
2. Description of the Prior Art
Active research and developments have been made in recent years on ferroelectric memories using ferroelectric capacitors, dynamic random access memories (DRAMs) using high-dielectric-constant capacitors, etc. These ferroelectric memories and DRAMs contain selective transistors, and each capacitor connected to one diffusion layer of each selective transistor functions as a memory cell and stores information. Ferroelectric capacitors use, as the capacitive insulating film, a ferroelectric film composed of Pb(Zr,Ti)O3 (hereinafter referred to as PZT) or the like, and can store non-volatile information by polarizing the ferroelectric film. Meanwhile, high-dielectric-constant capacitors use, as the capacitive insulating film, a high dielectric-constant thin film composed of (Ba,Sr)TiO3 (hereinafter referred to as BST) or the like, and accordingly can show a high capacitance and can be produced in a fine structure. When such a ceramic material is used in semiconductor capacitors, it is highly important that the ceramic material deposited on a lower electrode is divided electrically in order to obtain fine capacitors.
As the method for forming a thin ceramic film by deposition, there have been reported a sol-gel method, a sputtering method and a CVD method.
In the sol-gel method, organometal materials dissolved in an organic solvent are coated on a wafer having lower electrodes formed thereon, by spin coating, and the coated materials are subjected to crystallization by annealing in oxygen. In the method, the temperature needed for crystallization is very high because the crystallization takes place in a solid phase; and in the case of PZT, the crystallization temperature necessary for] achieving sufficient ferroelectric properties is 600xc2x0 C. and, in the case of BST, the crystallization temperature for achieving sufficient high dielectric-constant properties is 650xc2x0 C. In the above method, there is a problem that the orientation of the crystals formed is non-uniform. Furthermore, the sol-gel method is difficult to apply to wafers of large diameter because it is inferior in coatability on surface of different levels, and is not suitable for high integration of device.
In the sputtering method, reactive sputtering is conducted using a sintered ceramic target of the same material as the film to be formed and an (Ar+O2) plasma. A film is formed on a wafer having electrodes, and then the film is subjected to crystallization by annealing in oxygen. A uniform film is obtained by using a large-diameter target and a sufficient film formation rate is obtained by using a large power for plasma generation. In the sputtering method as well, however, a high temperature is needed for crystallization; in the case of PZT, the crystallization temperature for achieving sufficient ferroelectric properties is 600xc2x0 C. and, in the case of BST, the crystallization temperature for achieving sufficient high-ielectric constant properties is 650xc2x0 C. Further in the sputtering method, since the composition of the film obtained is decided substantially by the composition of the target used, a change in film composition requires a change of target, which is disadvantageous from the operational standpoint.
In the CVD method, a raw material mixture of gaseous state is transferred into a vessel accommodating a heated substrate to form a film therein. The CVD method is superior in film uniformity on large-diameter wafer as well as in coatability onto surface of different levels, and is considered to be promising as a technique enabling mass production of ULSI. The metals constituting the ceramic film formed include Ba, Sr, Bi, Pb, Ti, Zr, Ta, La, etc.; however, the hydrides or chlorides thereof appropriate for use as raw materials of CVD are very few in kind and, therefore, organometals of such metals are used in the CVD. These organometals, however, have low vapor pressures and are mostly solid or liquid at room temperature and are transferred by using a carrier gas.
In conducting such transfer, however, it is difficult to quantify the flow amount of organometal gases in carrier gas and accurately control the flow amount of organometal gases. It is because the carrier gas contains organometal gases in an amount larger than the amount corresponding to the saturated vapor pressure determined by the temperature of the raw material tank and because the flow amount of organometal gases is influenced not only by the flow amount of carrier gas but also by the surface area of solid raw materials, the temperature of raw material tank, etc. According to the description on the formation of a film of PTO (lead titanate, PbTiO3) by CVD, in Jpn. J. Appl. Phys. Vol. 32 (1993) p. 4175, the temperature of formation of PTO film is very high (570xc2x0 C.) and the crystal orientation in film is not uniform.
In conventional production of ferroelectric memories or DRAMs, the dielectric film of capacitor has been formed by the above-mentioned methods. In these methods, however, the heating at high temperature such as about 600xc2x0 C. or more in oxygen atmosphere is essential and the control of crystal orientation has been difficult.
Description is made on the structure of semiconductor device. In order to allow each ferroelectric capacitor or high-dielectric-constant capacitor of semiconductor device to function appropriately, it is necessary that either electrode of the capacitor is electrically connected to the diffusion layer of selective transistor. Conventional DRAMs have generally employed such a capacitor structure that a polysilicon connected to one diffusion layer of selective transistor is used as a electrode and a thin capacitive insulating film such as SiO2 film, Si3N4 film or the like is formed on the polysilicon. However, since the above thin ceramic film is an oxide, the polysilicon is oxidized when the film is formed directly on the polysilicon, and it is impossible to form a good ceramic thin film. Hence, p. 123 of 1995 Symposium on VLSI Technology Digest of Technical Papers describes a cell structure in which the upper electrode of capacitor and the diffusion layer are connected by local wiring of a metal such as Al or the like. Further, p. 843 of International Electron Devices Meeting Technical Digest, 1994 describes a technique of forming a PZT insulating film on a polysilicon using a TiN barrier metal with respect to DRAM, for example, p. 831 of International Electron Devices Meeting Technical Digest, 1994 describes a technique of forming a STO (strontinum titanate, SrTiO3) thin film as an insulating film on a RuO2/TiN lower electrode formed on a polysilicon plug, to form a capacitor.
In developing a fine capacitor structure, conventional methods for producing a ferroelectric film or a high-dielectric-constant film have had the above-mentioned problem that a high temperature is required for crystallization of insulating film and a problem associated with etching, described below.
High-dielectric-constant or ferroelectric capacitors have hitherto been produced as follows. First, as shown in FIG. 17(A), element-isolating regions 111, source/drain regions 112, gate electrodes 113, etc. are formed on a semiconductor substrate 110. Then, as shown in FIG. 17(B), an inter-layer insulating film 114 is formed by deposition and metal plugs 115 are formed, after which a lower electrode film 101, a capacitive insulating film 102 and an upper electrode film 103 are formed in this order. Thereafter, dry etching is conducted from the upper electrode film to the lower electrode film to form capacitors separated from each other.
As the material for the lower electrode of high-dielectric-constant or ferroelectric capacitor, a noble metal (e.g. Pt, Ir or Ru) or an oxide thereof is used in many cases. The etching of the lower electrode film is generally conducted by milling because the above material is difficult to etch by reactive etching. The etching of the lower electrode film is conducted in a state that the end of the capacitive insulating film 102 is exposed. If, after the steps up to FIG. 17(B), etching is conducted in a vertical direction, the etching residue of lower electrode film appears owing to milling, during the etching of the lower electrode film; this etching residue adheres to the exposed end of the capacitive insulating film 102. It is pointed out that this adhesion causes the short-circuiting of capacitor.
Hence, in order to prevent the short-circuiting of capacitor, the etching after the steps up to FIG. 17(B) is generally conducted by, as shown in FIG. 17(C), etching the end of the capacitive insulating film by sputtering, so that the capacitors after etching has a tapered surface 104. In this etching, however, the capacitive insulating film inevitably has a large etching damage at the end thereof. As the capacitive area is smaller (the capacitor is finer), the ratio of the damaged area to the total capacitive area is larger; therefore, the deterioration of capacitance due to etching becomes a serious problem. The etching into a tapered side surface is not suitable for production of small capacitors.
Hence, the present applicant proposed, in Japanese Patent Application No. 11-053239, a process for production of capacitor, which comprises first etching a lower electrode film 105 to form lower electrodes of desired shape, as shown in FIG. 18(A), and then, as shown in FIG. 18(B), forming a capacitive insulating film 106 and an upper electrode film 107, followed by etching only the upper electrode film to form upper electrodes of desired shape. In such a capacitor structure, the etching of the end of the capacitive insulating film 106, such as shown in FIG. 17(C), is unnecessary and the above-mentioned etching damage at the end of the capacitive insulating film can be prevented. Further, the upper electrodes can be formed as a plate wiring; therefore, it is unnecessary to form a plate wiring on each upper electrode as done in conventional capacitors, and a reduction in production cost is made possible. Furthermore, the deterioration of ferroelectric film appearing during the formation of plate wiring can be minimized. Moreover, no tapering is necessary during etching, and the process is suitable for production of fine capacitors.
In this process as well, however, a high crystallization temperature of 600xc2x0 C. or more is necessary as long as the capacitive insulating film is formed by the sol-gel method or by the sputtering method. Use of such a high temperature results in a reaction, at the exposed portion of silicon oxide film where no capacitor is formed, between the metal constituting the capacitive insulating film and the silicon oxide film. This incurs insufficient insulation between lower electrodes separated from each other. Further, the metal diffuses through the silicon oxide film, which significantly deteriorates the properties of the transistors formed below.
Meanwhile, the present inventor proposed a method where no high crystallization temperature is required, i.e. in Japanese Patent Application No. 10-219184 (not yet laid-open at the filing of the present application), a CVD method for forming a metal oxide dielectric film at a temperature of 450xc2x0 C. or less. With this method, the reaction between the film metal and the exposed portion of silicon oxide film can be prevented and the above-mentioned problem can be solved.
After further investigation, however, the present inventor learned that when lower electrodes of desired shape are formed on a silicon oxide film and then a capacitive insulating film is formed thereon by CVD using organometal raw materials, there appear some cases where no crystallization takes place on each of the finely separated lower electrodes in a portion of areas of capacitive insulating film. A ceramic insulating film shows excellent properties such as high dielectric constant, ferroelectricity and the like only when crystallized; therefore, unless crystallization takes place, no sufficient capacitance or spontaneous polarization is obtained on fine capacitor electrodes.
To further improve the above technique, the present invention aims at providing a semiconductor device having capacitor elements which are superior in properties and reliability even when the capacitor elements are produced in a fine structure.
The present invention also aims at providing a semiconductor device having capacitor elements superior in noise resistance.
The present invention also aims at providing a process for producing a semiconductor device having capacitor elements which are superior in properties and reliability even when the capacitor elements are produced in a fine structure, by reliably conducting the crystallization of the metal oxide dielectric film of capacitor particularly into a perovskite type crystal at a low temperature.
The present invention also aims at providing a process for producing a semiconductor device having capacitor elements superior in noise resistance.
Accordingly, the present invention is directed to a semiconductor device comprising an array of a plurality of capacitor elements which are each a laminate of a lower electrode, a metal oxide dielectric film of a perovskite type crystal represented by ABO3 and an upper electrode in this order on a semiconductor substrate;
the semiconductor device comprising a crystallization-assisting conductive film made of a material capable of catalyzing the formation of active precursors of the metal oxide dielectric film in formation of the metal oxide dielectric film; the crystallization-assisting conductive film being formed outside the array at a distance not larger than the diffusion distance of active precursor from the outermost lower electrodes in the capacitor array, and in a width at least equal to the width enabling formation of the crystal nuclei of the metal oxide dielectric film.
The present invention is also directed to a semiconductor device comprising an array of a plurality of capacitor elements which are each a laminate of a lower electrode, a metal oxide dielectric film of a perovskite type crystal represented by ABO3 and an upper electrode in this order on a semiconductor substrate;
the semiconductor device comprising a crystallization-assisting conductive film made of a material capable of catalyzing the formation of active precursors of the metal oxide dielectric film in formation of the metal oxide dielectric film; the crystallization-assisting conductive film covering at least 10% of the area of the semiconductor device.
In the present invention, the crystallization-assisting conductive film and the lower electrodes may be formed on the surface(s) of the same or different heights.
In each of the above cases, the crystallization-assisting conductive film is preferably formed on a surface which is exposed right before the formation of the metal oxide dielectric film.
The present invention is further directed to a process for producing a semiconductor device comprising an array of a plurality of capacitor elements which are each a laminate of a lower electrode, a metal oxide dielectric film of a perovskite type crystal represented by ABO3 and an upper electrode in this order on a semiconductor substrate; the process comprising steps of:
forming the lower electrodes and a crystallization-assisting conductive film on an insulating film; both films being independently made of conductive material, where the crystallization-assisting conductive film being a material capable of catalyzing the formation of active precursors of the metal oxide dielectric film in formation of the metal oxide dielectric film; the crystallization-assisting conductive film being formed outside the capacitor array area at a distance not larger than the diffusion distance of active precursor from the outermost lower electrodes of the array, in a width at least equal to the width enabling formation of the crystal nuclei of the metal oxide dielectric film; and
forming the metal oxide dielectric film on the formed lower electrodes and the formed crystallization-assisting conductive film.
Yet another aspect of the present invention is directed to a process for producing a semiconductor device comprising an array of a plurality of capacitor elements which are each a laminate of a lower electrode, a metal oxide dielectric film of a perovskite type crystal represented by ABO3 and an upper electrode in this order on a semiconductor substrate; the process comprising a step of:
forming, on an insulating film on which the lower electrodes are to be formed, a crystallization-assisting conductive film so as to cover at least 10% of the area of the semiconductor device; the crystallization-assisting conductive film being made of a material capable of catalyzing the formation of active precursors of the metal oxide dielectric film in formation of the metal oxide dielectric film.
In the process of the present invention, it is preferred that one conductive material is used for formation of the crystallization-assisting conductive film and the lower electrodes, a film of the conductive material is formed on the insulating film, and the film is etched to simultaneously form the lower electrodes and the crystallinity-assisting conductive film in respective patterns.
Also in the process of the present invention, it is preferred that the formation of the metal oxide dielectric film is conducted under first conditions which are initial conditions and then under second conditions which are subsequent conditions and the two conditions are different from each other.
In one method for practicing the above formation of the metal oxide dielectric film, under the first conditions for film formation, all kinds of the organometal material gases to become a material of the metal oxide dielectric film are used and an initial nuclei or initial layer of a perovskite type crystal structure is formed on the lower electrodes and the crystallization-assisting conductive film and, under the second conditions for film formation, the perovskite type crystal structure is allowed to grow on the initial nuclei or initial layer.
In other method for practicing the formation of the metal oxide dielectric film, under the first conditions for film formation, only some kinds of the organometal material gases to become a material of the metal oxide dielectric film are used and an initial nuclei or initial layer of a perovskite type crystal structure is formed on the conductive material and, under the second conditions for film formation, the perovskite type crystal structure is allowed to grow on the initial nuclei or initial layer.